If you see someone driving a convertible with the top down on the freeway this summer, you might shout a question at them above the roar of traffic: How’s your health insurance policy?

Driving on congested roadways is unhealthy for all of us, but especially for those who fail to maintain a strong barrier between the cabin and the many toxic emissions spewing out of everyone’s vehicles. The best way to keep air pollution out is to keep the roof on, the windows rolled up, and push that “recirculate” button on the vent control panel.

“If you’re concerned about trying to reduce your exposure, which probably anyone in L.A. who drives very much at all should, it’s not good to have your windows rolled down,” said Scott Fruin, an assistant professor in preventive medicine at the University of Southern California’s Keck School of Medicine. “The one exception might be if you have a really old car and it’s just too hot, you don’t have AC or something. But I would say in general, health-wise it’s much better to have your windows rolled up and on recirculate, as much as you can do it.”

Two years ago, Fruin and his USC colleague, research assistant Neelakshi Hudda, published a study in Environmental Science & Technology, showing that using the recirculate button — the one that usually has the round arrow inside it — can reduce the driver’s level of exposure to outside air pollution to 20 percent of on-road levels.

The researchers used a mobile testing vehicle to measure the contaminants coming from tailpipes on some of SoCal’s busiest freeways and other busy roads. The study mostly covered exposure to particulate matter (also called soot), and ultra-fine particles. These are molecules so small that they can actually enter the bloodstream and interrupt the function of individual cells.

Data collected by the team since then has shown the benefits of air recirculation for larger particles, including PM2.5, so named because they’re 2.5 micrometers in diameter.

Of all the tiny pollutants, the science on PM2.5 is the strongest, Fruin said, because it’s easier for instruments to detect.

“With that science, they found a lot of relationships to cardiovascular disease and premature mortality, so that’s what you see in the media now that’s killing 7 or 8 million people worldwide every year, especially in the developing-country cities.”

Air quality isn’t just an issue in developing nations, however: According to the American Lung Association’s 2015 State of the Air report, more than 4 in 10 Americans live in counties where the air is unhealthy to breathe. Vehicle emissions account for about half the air pollution in the United States, and the problem is much worse in urban areas than rural ones.

One of the solutions Fuel Freedom is fighting for is for the increased availability of cheaper, cleaner transportation fuels at the pump, so consumers can have a choice besides gasoline. Wider adoption of those alternative fuels would reduce the level of toxic compounds being released into the air.

Fruin and his team are quick to note that using the recirculate button for prolonged periods, especially if there’s more than one person in the car, can make the air inside stuffy, owing to the increased level of carbon dioxide exhaled by the occupants. Every 10 or 15 minutes, allow outside air — we won’t call it “fresh” air — to come in.

Of course, filtering out pollution only really takes care of the particles. It can’t stop gaseous toxics from coming in.

“So all the pollutants like nitrogen dioxide or some of the volatile organic carbons like benzene, you’re gonna get a high exposure pretty much no matter what in a car, especially on freeways or in congestion, unless you have a very specialized filter system. … So it’s still important for people to drive less.”

Older cars tend to be more “leaky” with outside air than newer ones. A good rule of thumb to remember — and this goes for houses that are near busy roadways as well — is that if you can hear the noise of traffic, those emissions probably are coming in as well.

“If traffic is loud, you should try to make your home quieter, because that seals it up,” he said. “The same with cars: If you get a lot of traffic noise while you’re driving, chances are your car is leakier.”